Applications for bonding layers of wafer-shaped samples are known from the article entitled “Adhesive Wafer Bonding” by F. Niklaus, G. Stemme, J.-Q. Lu and R. J. Gutmann published in the Journal of Applied Physics, Applied Physics Reviews—Focused Review, Vol. 99, No. 1, pp. 031101.1 to 031101.28, 2006. The bonding layers used are usually polymer adhesive layers that are applied to one or both of the wafer-shaped samples to be bonded. Once the surfaces of the samples covered with the polymer adhesive have been bonded, pressure or force is applied to the wafer-shaped samples. A temperature treatment is also typically used to reinforce the bonding process.
The bonding and production of a bonding layer between wafer-shaped samples is thus a relatively simple, robust and inexpensive process, wherein the bond strength and the formation of cavities on the surfaces to be bonded can be influenced by the type of polymer adhesive used. The quality of the bonding layer is determined by the degree of polymerisation of the polymer adhesive, the wafer-shaped sample material, the size of foreign particles on the wafer surface and the wafer topography, the polymer thickness, the polymer viscosity during production of the bonding layer and the bond pressure applied to the two wafer-shaped samples. While foreign particles in the bonding layer can be tolerated as long as they remain smaller than the thickness of the bonding layer, cavities and adhesive interruptions within the bonding layer cannot be tolerated and can lead to rejection.
In some applications in semiconductor technology, for example, silicon wafers with a thickness of the order of a few tens of μm (micrometers) and diameters of over 10 inches or 250 mm are produced and processed. Such silicon wafers behave like an aluminium foil, for example, and cannot be handled or processed using standard tools. Such wafer-shaped samples warp in such a manner that they cannot even be transported or stored in a wafer cassette. For this reason thin wafer-shaped samples of this type are processed using the bonding layer on a carrier wafer, a thin functional wafer thus being held and stabilised by a stable carrier waver using the bonding layer.
In manufacturing thin wafers of this type as functional wafers it has proved useful to grind the functional wafer down to a thickness of a few tens of μm as indicated above. Once the functional wafer with its finished functional face is bonded onto the carrier wafer, a composite comprising a bonding layer and wafers is held together during grinding by means of the bonding layer. During grinding in particular bonding layer regions with reduced adhesive layer thickness or with interruptions or air bubbles in the adhesive layer are extremely critical since it is here that mechanical stresses capable of destroying the functional wafer can occur. For this reason it is advantageous to examine the entire surface of the adhesive layer for cavities and defects to an accuracy of less than one millimeter prior to grinding.
To this end wafer-shaped samples with the aforementioned diameter of, for example, more than 10 inches can be point scanned with a spiral motion. At a rate of 4 kHz and a step width of 1 mm with a diameter of 300 mm this requires a test time of approximately 70 seconds. The measurement is preferably made through the carrier wafer because the bonding layer cannot be measured reliably through the functional wafer using an optical method due to unevennesses in the structure, due to the inhomogeneity of light absorption caused by doping substances and due to metallised layers on the functional wafer used as the wafer-shaped sample.
A measuring head configured for an OCT (Optical Coherence Tomography) process, in particular for an FD-OCT (Frequency or Fourier Domain Optical Coherence Tomography) process, is known from publication WO 2006/028926 A1.
A measuring head for the OCT process with a reference arm or a reference plane is able to record the absolute distances between a reference face and any surface on a measurement object as well as measuring the layer thickness. A rotating disc positioned in the reference arm such that it is able to rotate has proved useful for this purpose because with this partially coated disc it is possible to achieve phase shift. However, this disc provides no support for the attribution and classification of measured layer thickness peaks and distance peaks which cannot be clearly classified as layer thickness measurements or distance measurements without prior knowledge of the layer structure of a measurement object.